A virtual tape device is known which imaginarily behaves as a tape library device for a host computer having requested the device to read or write data.
The virtual tape device reads or writes data (logical volume) practically by using a disk array device. As the virtual tape device uses the disk array device which allows quicker data access than a magnetic tape so as to reply to the host, the virtual tape device performs a process at a high speed in comparison with a case where only a tape library device is used.
Then, the virtual tape device performs a process for saving in the tape (migration) the data written in the disk array device in the background without participation of the host. As saving the data, the virtual tape device can perform a saving process for copying the same data into two tapes, which is called a dual-save process.
According to the dual-save process, the same data is saved severally in the two tapes. If the one tape is in failure, the data can be read from the other tape. Thus, certainty of the data can be enhanced.
According to the dual-save process, if both writing processes into the two tapes finish, a writing process into a back end is regarded as having finished, and the same data existing in the disk array device that is used as a cache can be deleted.
In a case where only the writing process into the one tape delays owing to a load on the library device or a failure, even if the writing process into the other tape has finished, the writing process into the back end has not finished yet. Thus, the disk array device has to keep the data.
If such an event frequently occurs, the disk array device is pressed by the data which cannot be deleted, and ends up in a state where new data coming from the host computer cannot be written. Thus, if the data increases in size in the disk array device and a remaining capacity of the disk array device is smaller than a certain capacity, a tape library of a highest data writing speed is chosen. A method is known for moving to a single-save process then, saving the data in a tape of the chosen tape library device and writing the data from the tape into the other tapes, as disclosed in Japanese Laid-open Patent Publication No. 2008-123444, and No. 2008-77519.